KR101143177B1 - Method for Resistance Spot Welding of Plating Steel - Google Patents

Abstract

A resistance spot welding method of a plated steel material capable of reducing surface cracking of a welded portion is disclosed. First, an electrode is pressed against a plated steel for a first time to bring the electrode into contact with the plated steel, a preliminary current pulse having a first magnitude is applied to the electrode for a second time, the plated steel is cooled for a third time, After the welding current pulse having the second magnitude is applied to the electrode for the fourth time, the application of the welding current pulse is stopped and the pressing force of the electrode against the plated steel is maintained for the fifth time. Therefore, even when the main welding is performed in a current range in which the discharge phenomenon occurs, the surface crack of the welded portion can be reduced.

Description

METHOD FOR RESISTANCE SPOT WELDING OF PLATED STEEL

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of welding a plated steel material, and more particularly, to a resistance spot welding method of a steel material applicable to an automotive coated steel plate.

Currently, the types of galvanized steel sheets that are mainly used for the exterior of a vehicle include galvanized steel (hereinafter referred to as "GI steel") and galvannealed steel (hereinafter referred to as "GA steel" ).

After passing through the plating bath, the GI steel sheet in the manufacturing process is formed of pure Zn, and the GA steel sheet is formed of an Fe-Zn based intermetallic compound in the plating layer due to the alloying reaction between iron and zinc. In general, GI steel has excellent formability and corrosion resistance, and GA steel has better weldability and paintability than GI steel.

In addition, resistance spot welding is widely used in automobile production process because it is easy to automate and is suitable for mass production process. Recently, as the demand of GI steel plate has increased, It is becoming a concern.

1 is a schematic view showing the principle of general resistance spot welding.

1, when a large current (i) in units of kA is applied in a state where a pressure P is applied between the upper and lower portions of the metal materials 13 and 14 placed between the two electrodes 11 and 12, Heat is generated due to the contact resistance and the intrinsic resistance of the metal generated on the contact surfaces (b, d, f) of the metal wires 13 and 14 and the metal wires 13 and 14, . Here, the total amount of heat input during the welding process is proportional to the welding current (i), the electric resistance (R) and the welding time (t) by the Joule's law (Q = i ² Rt).

Particularly, the heat input is proportional to the square of the welding current (i ² ), so that if the welding current (i) becomes excessively large, surface expulsion of molten metal occurs on the surface of the metal material being welded.

The basic process parameters of resistance spot welding include welding current, electrode pressure and welding time, but the heat input due to resistance heat is most influenced by the welding current among the above process variables. That is, when the electrode pressure and the welding time are fixed and the welding current is gradually increased among the process variables, the amount of heat input is correspondingly increased, so that the size of the nugget as the welding metal produced by melting and solidification grows.

The nugget diameter is an important factor that determines the quality of the spot weld. In order to achieve the desired welding, it is necessary to avoid the discharge phenomenon and to obtain the optimum width (for example, 4√T or more, where T is the thickness of the material) Welding shall be carried out. However, in a typical automobile body assembly line, welding is performed in a current range in which a discharge phenomenon occurs because a larger welding size is secured and a high current is applied to the worker to obtain a visual sense of comfort.

Particularly, in the case of a plated steel sheet having a zinc layer on its surface as in a zinc-plated steel sheet, cracking may occur on the surface of the welded portion when welding is performed under a condition of a current range over which a discharge phenomenon occurs, And the deterioration of the corrosion characteristics is disadvantageous.

Fig. 2 shows a surface crack inspection method of the spot welded portion of the galvanized steel sheet.

Referring to FIG. 2, first, the presence or absence of surface cracks is confirmed through a spot magnifying microscope (stereomicroscope) after spot welding of a galvanized steel sheet.

In the case where surface cracks are present, as shown in FIG. 2, cut and polished in a direction perpendicular to the surface where the surface cracks are conspicuous (that is, in the direction of I-I '), Observe the cut surface.

As shown in Fig. 2, it can be seen that the remarkable surface crack progresses inwardly even in the cross section. In this way, surface cracks in spot welds can be confirmed macroscopically through external inspection and cross-section observation.

FIG. 3 is a cross-sectional view of a region where surface cracking occurs in a spot welded portion of a galvanized steel sheet. In a spot welded portion of a galvanized steel sheet having a high strength TRIP (Transformation Induced Plasticity) That is, spot welding is performed at 12.0 kA, which is a current higher than 10.4 kA, which is the current causing discharge phenomenon).

The results of observing the cut surface of the spot welding with surface cracks by optical microscope showed that cracks progressed from the surface layer of the concave portions (a and b) and the inclined portion (c) . In particular, it can be seen that the cracks generated in the center of the concave part propagated to the nugget along the grain boundaries in the surface layer.

The cause of the surface cracking of the welded portion as shown in Fig. 3 is excessive welding current, increase in welding time, insufficient pressing force, inadequate electrode tip diameter, and electrode misalignment.

Therefore, there is a need for a welding method capable of preventing the surface cracking of the welded portion even at a current point exceeding the current range causing discharge phenomenon at the point of spotting of the galvanized steel sheet.

It is an object of the present invention to solve the problems described above and provide a resistance spot welding method of a plated steel material capable of reducing surface cracks in a weld portion when performing welding in a current range in which discharge phenomenon of molten metal occurs.

The technical objects of the present invention are not limited to the technical matters mentioned above, and other technical subjects not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention, there is provided a method of welding a plated steel material to a plated steel material, the method comprising: pressing an electrode onto a plated steel material for a first time period to bring the electrode into contact with the plated steel material; Applying a preliminary current pulse having a first magnitude to the electrode for a second time after contacting the plated steel material; cooling the plated steel material for a third time after applying the preliminary current pulse; Applying a welding current pulse having a second magnitude to the electrode for a fourth time after the plated steel has cooled and stopping application of the welding current pulse and forcing the pressing force of the electrode against the plated steel during a fifth period of time .

The current magnitude of the preliminary current pulse having the first magnitude may have a magnitude corresponding to a range of 80 to 95 percent of the magnitude of the current magnitude of the welding current pulse having the second magnitude.

The third time at which the plated steel is cooled may be set to be at least twice the second time at which the preliminary current pulse is applied.

Wherein the plated steel is a galvanized steel, the current magnitude of the preliminary current pulse is greater than or equal to 10.4 kA, and the current magnitude of the welding current pulse may be set greater than or equal to 11 kA.

The plated steel is a galvanized steel, the preliminary current pulse is applied for a period of time not exceeding three cycles, and the plated steel may be cooled for a period of time greater than six cycles.

The current magnitude of the preliminary current pulse and the time for which the preliminary current pulse is applied may be set to a current magnitude within a range such that melting and discharge of the plating layer occurs earlier than the steel material in the plated steel and an application time.

According to the resistance spot welding method of a plated steel material as described above, a preliminary current pulse having a predetermined magnitude is applied for a predetermined time before applying the welding current pulse for welding, and the magnitude corresponding to twice the time For a predetermined period of time to pre-melt the plating layer, thereby facilitating melting and discharging of the plating layer. Therefore, even if the welding is performed in the current range where the discharge phenomenon occurs, the surface crack of the welded portion can be reduced.

1 is a schematic view showing the principle of general resistance spot welding.
Fig. 2 shows a surface crack inspection method of the spot welded portion of the galvanized steel sheet.
3 is a cross-sectional view of a portion where surface cracks have occurred in the spot welded portion of the galvanized steel sheet.
4 is a flowchart showing a resistance spot welding method of a plated steel material according to an embodiment of the present invention.
5 is a graph showing a current application pattern in a resistance spot welding method of a plated steel material according to an embodiment of the present invention.
6 is a graph showing the simulation results of the pre-melting conditions of the hot dip galvanized layer.
7 is a front view and a cross-sectional view of a spot welded portion using a resistance spot welding method of a plated steel material according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Also, the terms used in the present application are used only to describe certain embodiments and are not intended to limit the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In order to facilitate the understanding of the present invention, the same reference numerals are used for the same constituent elements in the drawings and redundant explanations for the same constituent elements are omitted.

Hereinafter, a resistance spot welding method of a galvanized steel material will be described and illustrated by way of example in a method of resistance spot welding of a plated steel material according to an embodiment of the present invention. However, this is only one example for the concrete implementation of the present invention, and it goes without saying that the resistance point welding method of the plated steel material described below can be applied to various kinds of plated steel materials other than the zinc plated steel material.

Hereinafter, the principle of surface cracking as a theoretical background of the resistance spot welding method of the plated steel material according to the embodiment of the present invention will be described.

First, a liquid metal embrittlement (LME) phenomenon similar to the surface cracking mechanism of the spot welded portion will be described.

LME refers to a phenomenon in which brittle fracture occurs due to abrupt decrease in ductility when a soft metal or alloy is under stress in contact with a liquid metal having a thickness of a few μm. Full-scale research on LME began with the publication of several research papers since the 1950's, and the developmental mechanisms and influencing factors for LME were investigated.

Two prerequisites are necessary for the LME to occur. First, there must be sufficient working stress to cause plastic deformation. Here, the working stress can be a complex stress state as well as a simple tensile stress, but there is no occurrence in a compressive stress state. Second, the liquid phase in contact with the solid phase to which the stress acts should be in close contact with the atomic distance. That is, the liquid phase should easily penetrate when cracks occur and propagate.

 The fracture by LME is mostly intergranular fracture, but in the absence of grain boundary, fracture may occur along the transgranular. The two classic models explaining the failure mechanism by LME are as follows.

(1) Reduction in surface energy model

 It is hypothesized that the surface energy reduction model is related to the reduction of surface energy of solid metal such as adsorption of liquid metal or surface tension between solid phase and liquid phase. Cracking occurs in principle along the grain boundary. With respect to intergranular invasion, the relationship of Equation (1) holds when the surface energy of the solid-liquid interface and the surface energy of the solid-phase interface maintain equilibrium in the crystal grain boundaries.

는 입계의 계면에너지를 의미하고,

는 고상-액상간의 계면에너지를 의미하며, θ는 2면각으로

에 의해 0도 내지 180도 사이의 일정한 값을 가진다. 만약

이 보다 작은 경우에는 2면각은 0도가 되고 용융금속은 응력이 존재하지 않을 때에도 고체금속의 입계로 침입한다. 또는

이

보다 큰 경우에는 2면각은 0도 보다 큰 각을 가지게 되고, 응력이 존재하지 않을때는 무제한 입계로의 침입은 불가능하게 된다.In Equation (1)

Means the interfacial energy of the grain boundary,

Refers to the interfacial energy between the solid phase and the liquid phase, and?

And has a constant value between 0 and 180 degrees. if

In the case of smaller values, the biaxial angle becomes 0 degrees and the molten metal penetrates into the grain boundary of the solid metal even when no stress is present. or

this

, The biaxial angle has an angle larger than 0 degree, and when there is no stress, infiltration into the unlimited grain boundary is impossible.

As mentioned above, the LME promotes penetration of the liquid metal by the tensile stress, resulting in brittle fracture. Under static conditions, simple intergranular penetration should be considered as another phenomenon.

(2) Adsorption-induced reduction in cohesion model

The adsorption-induced cohesion reduction model is that the cause of embrittlement is due to the local reduction of the interatomic bond strength due to the liquid element chemically adsorbed on the solid surface or the tip of the crack. Crack propagation begins by interatomic bond breakage and similar interatomic cracks occur in the propagating crack tip by successively chemically adsorbed elements. For example, suppose that the liquid atoms in the crack tip reduce the bond strength between the high-order atoms. The chemical adsorption causes electromagnetic rearrangement between atoms, which weakens the interatomic bonding force of the crack tip. Liquid atoms are easily chemically adsorbed on the newly formed crack surface and eventually complete destruction will occur if this relationship continues continuously.

In addition, the main factors affecting LME are as follows.

(1) Grain size

 If the size of the crystal grains is large, even if there is microcracks, breakage easily occurs. Therefore, in order to prevent LME, it is effective to minimize the size of the crystal grains to make the crystal grains finer.

(2) Temperature

The LME generally begins to rise on the melting point of the liquid metal causing the brittleness, and this temperature is the ductile-brittle transition temperature, and the tendency of embrittlement becomes more pronounced when the temperature is further increased.

(3) Prestrain and cold working

The sensitivity of LME increases in the presence of pre-strain or initial cold working, but the sensitivity of LME decreases gradually as the amount of cold working increases, indicating the tendency of ductile fracture as well as the shape of fracture. The reason is that as the amount of cold working increases, a fibrous crystal structure is formed, resulting in a number of complicated crack paths.

(4) Elemental addition in liquid metal

The addition of alloying elements in the liquid metal can prevent or even aggravate LME. For example, the sensitivity to LME increases due to the segregation of impurity alloying elements in the grain boundaries in high-strength steels, and the transformation of austenite to ferrite due to local Ni consumption at grain boundaries due to the formation of Zn-Ni alloy phase in stainless steels And volume expansion can also cause high stresses and cracks.

(5) metallurgical conditions of solid metal

Since LME breaks mostly along the grain boundary, it can be influenced greatly by the grain size, structure, and chemical characteristics of the grain boundary. It is also argued that the alloying of elements that normally reinforce materials has the effect of promoting LME.

 Recently, it is widely believed that the surface cracking of the spot welded part of the galvanized steel sheet in the transformation-strengthening high strength steel such as AHSS (Advanced High Strength Steel) for automobiles is a kind of LME. The aspect of crack propagation at this time is that the low melting point metal on the surface during fusion welding penetrates into the grain boundaries of the weld heat affected zone, so that the residual stress and thermal stress act during the solidification process and the grain orientation proceeds.

Surface cracking in spot welding is also caused by poor welding such as electrode misalignment, insufficient electrode cooling, Excessive electrode wear, and Excessive heat input. .

Among the causes of surface cracks, the cause of surface cracking is most likely caused by an excessive heat input. That is, when the amount of heat input is excessively large, cracks may be promoted due to the reaction of Zn in the surface plated layer and Cu in the electrode. For example, Zn-Cu phases with different compositions are produced according to the Zn content in the range of Zn content of 40% (at.%) Or more in the binary state diagram of Zn-Cu, (Cu5Zn8, Zn / Cu = about 1.6) is known as a hard and weak phase having a complex crystal structure and a hardness of about 360 to 430 Hv.

As described above, the cause of the surface cracking of the welded portion is excessive welding current, increase of welding time, insufficient pressing force, undersized tip of the electrode, misalignment of the electrode, and the like. And it is a passive method for suppressing the surface cracking, and it is difficult to obtain practical effect in practice.

Therefore, in the resistance spot welding method of the plated steel material according to the embodiment of the present invention, in consideration of the zinc penetration into the material and the electrode reaction during the welding of the high strength steel galvanized steel sheet and the thermal deformation characteristics due to the tensile stress condition in the surface layer, So that zinc having a relatively low melting point present on the surface of the material during welding is removed in advance to prevent surface cracking of the welded portion.

FIG. 4 is a flow chart showing a resistance spot welding method of a plated steel material according to an embodiment of the present invention, and FIG. 5 is a graph showing a current application pattern in a resistance spot welding method of a plated steel material according to an embodiment of the present invention . 6 is a graph showing the simulation results of the pre-melting conditions of the hot dip galvanizing layer.

Referring to FIGS. 4 to 6, a resistance spot welding method of a plated steel material according to an embodiment of the present invention will be described. First, the electrode is pressed against a galvanized steel during a first predetermined time T1, To be brought into contact with the galvanized steel (step 110).

Then, a current pulse (preliminary current pulse) having a predetermined first size A1 is applied to the electrode for a second time T2 to energize the galvanized steel (step 120). Here, the current pulse having the first magnitude A1 may vary depending on the kind of the plated steel and the thickness of the plated steel, and may generally be in the range of 80 to 95% of the welding current pulse amplitude A2. 6, the magnitude of the preliminary current pulse and the duration of the preliminary current pulse are set to a current magnitude and a current application time such that melting and discharge of the zinc plated layer occurs before the steel material in the galvanized steel Can be set. By the application of the preliminary current pulse as described above, the plating layer is melted and discharged easily before the concave traces shown in FIG. 2 are enlarged to a predetermined size or more, so that the plating layer is continuously melted along with the growth of the concave- Cracks can be prevented.

Thereafter, the galvanized steel material is cooled without applying current for a third predetermined time T3 (step 130). Here, the third time (T3, cooling time) may be at least twice the second time T2 at which the current pulse having the first magnitude A1 is applied. For example, if the thickness of the galvanized steel is 1.6T, the second time T2 may be less than or equal to three cycles (where one cycle time is 0.02 seconds for 50Hz) and the third time T3 May be 6 cycles or more, twice the second time (T2).

Thereafter, the galvanized steel is spot welded (step 140) by applying a current pulse (welding current pulse) having a preset second magnitude A2 to the electrode for a fourth time T4 to energize the galvanized steel, .

Here, the magnitude of the current pulse (i.e., the welding current pulse) having the second magnitude A2 may vary depending on the kind of the plated steel material and the thickness of the plated steel material, and the surface expulsion of the molten metal occurs Current range. For example, if the type of plated steel is a galvanized steel and the thickness is 1.6T, the second size A2 may be 11kA or more, and in such a case, the current pulse having the first size A1 (I.e., the preliminary current pulse) may be greater than or equal to 10 kA.

Further, the energization time T4 of the welding current pulse may be different depending on the type and thickness of the plated steel, and may be the same as the energization time in general resistance spot welding.

Then, the application of the current pulse having the second magnitude A2 is stopped and the pressing force of the electrode is maintained for a fifth predetermined time T5 (step 150). Here, the fifth time T5 may be equal to the pressing force holding time in general resistance spot welding.

In the resistance spot welding method of the plated steel material according to the embodiment of the present invention as shown in Figs. 4 and 5, on the basis of the simulation result of the dissolution condition as shown in Fig. 6, And the energization time of the current were set.

7 is a front view and a cross-sectional view of a spot welded portion using a resistance spot welding method of a plated steel material according to an embodiment of the present invention.

As shown in FIG. 7, in the resistance spot welding method of the plated steel material according to the embodiment of the present invention, the melting and discharging of the plating layer is facilitated by using the preheating effect by application of the preliminary current pulse, It can be seen that surface cracking does not occur even when the present welding is performed in the current range.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (6)

Pressing the electrode onto the plated steel material for a first time to bring the electrode into contact with the plated steel material;
Applying a preliminary current pulse having a first magnitude to the electrode for a second time after the electrode contacts the plated steel;
Cooling the plated steel material for a third time after applying the preliminary current pulse;
Applying a welding current pulse having a second magnitude to the electrode for a fourth time after the plated steel has cooled; And
And stopping the application of the welding current pulse and maintaining a pressing force of the electrode against the plated steel during a fifth time period. The method according to claim 1,
Wherein the current magnitude of the preliminary current pulse having the first magnitude is within a range of 80 to 95 percent of the magnitude of the current magnitude of the second magnitude welding current pulse. The method according to claim 1,
Wherein the third time during which the plated steel is cooled corresponds to at least twice the second time during which the preliminary current pulse is applied. The method according to claim 1,
Wherein the plated steel material is a galvanized steel material, the current amplitude of the preliminary current pulse is 10 kA or more, and the current amplitude of the welding current pulse is 11 kA or more. The method according to claim 1,
Wherein the plated steel is a galvanized steel and the preliminary current pulse is applied for a time less than or equal to three cycles time and the plated steel is cooled for a time greater than six cycles. The method according to claim 1,
The current magnitude of the preliminary current pulse and the time for which the preliminary current pulse is applied are set to a current magnitude and a current application time within a range such that surface expulsion of the plating layer occurs before the steel material in the plated steel Of the plated steel.

Images